I just have the thatch laid over the roof frame. Layered enough the thatch will keep almost all the rain out. Being in NJ the winters kill the thatch so when I do replace it next it will be with artificial thatch and I'll put down a plywood base covered with tar paper.

What I'm thinking for the tiki roof to look as real as possible, but with the change of weather is a pain! This will be my final plan:
1. Lay thatch and attach to the roof framing (so when anyone is under the tiki bar, look up and see the thatch)
2. Lay pressure treated plywood with tar paper to seal
3. Layer thatch for the final covering

I attach a bamboo grid to bamboo uprights. The grids are apprx 18” square. I tie the thatch to the grids with jute- looks great from below. I replace the thatch every couple of years and haven’t had a problem with rain. I usually do my replacing in the fall before the rains arrive. Dried out , brittle thatch leaks like crazy
Cheers